This invention is concerned with the ignition of fuel and air mixtures within an inert atmosphere. As used herein the term "inert atmosphere" refers to an atmosphere unsuitable to support combustion, e.g. containing no more than 10% oxygen. It will be appreciated, for safety reasons, it is preferred that the inert atmosphere in accordance with the present invention contain no more than 5% oxygen and such other components as desired, e.g., N.sub.2, CO.sub.2, water vapor and hydrocarbons.
It is known that various closed gaseous systems operate with significant concentrations of unburned hydrocarbons in an inert high temperature gaseous atmosphere used for the processing of combustible materials. During start-up and perhaps, more significantly, during restart of a burner it is necessary to provide a means to effect the ignition of the burner without substantially altering the inert nature of the atmosphere during this transitory condition. As used herein the term "closed" refers to a system that is isolated from ambient conditions.
For example, one closed system is disclosed in U.S. patent application Ser. No. 08/029556, filed Mar. 11, 1993 and contains unburned hydrocarbons and is useful for increasing the thermal energy released during drying and gasification of coal by utilizing large volumes of continuously flowing streams of inert gas for convective heat transfer. Inert gas as used herein refers to a gas having generally less than 5% oxygen by weight.
Heretofore, inert gas streams have typically been produced using well known air separation technologies such as cryogenic distillation, membrane separation and pressure swing absorption. Although the known methods of producing inert gas streams for coal drying and mild gasification processes have been proven to perform satisfactorily in certain applications, these technologies are cost ineffective when considered for large processing needs like mild coal gasification, coke preheating and the like. Large mild coal gasification systems may range up to 8,000 square feet and may use 5,000 to 10,000 standard cubic feet of inert gas per hour per square foot of cross section for thermal treatment of coal and/or oil shale whether the coal and/or oil shale is to be dried or fractioned into solid and gaseous phase components.
A preferred method for recycling of an oxygen deficient gas stream used in the heat treatment of combustible materials includes a coal drying process and a mild coal gasification process employing, in combination, recycling of inert gas and renewal of both the inert gas components and the sensible energy. The preferred method requires precise control of inert gas chemistry, gas supply temperature and advanced flammable gas and vapor handling technology. As used herein, recycled inert gas means that the inert gas, i.e., nitrogen from air, carbon dioxide from combustion and water vapor from combustion or from the drying process are recycled within the system except for controlled venting. Similarly, as used herein, renewal of both the inert gas components and the sensible energy means that a portion of the recirculated gas stream is mixed with air in a combustion chamber to release heat and to oxidize a portion of combustible vapors and gases which result from the thermal process, and as may be picked up in the recycle process. The recycling and renewal of the inert gas and utilization of process derived low calorific inert gas facilitates the possibility of an economically feasible advanced heat treatment process for combustible materials such as gas permeable, solid carbonaceous materials.
It will be appreciated that it is difficult to ignite a pilot and/or a burner in an inert atmosphere so as not to contaminate the inert composition of the recirculated gases in either the coal drying process or mild coal gasification process. Furthermore, the presence of volatile combustible materials necessitates that the inert atmosphere within the system be maintained, especially during ignition or reignition to prevent the inadvertent combustion of the combustible materials.
Accordingly, one aspect of the present invention is to provide a method of igniting a flame in an inert atmosphere without substantially changing the inert nature of the gaseous atmosphere within the system or permitting significant entry of oxygen for unintentional ignition of the hydrocarbons contained in the system. The present invention is particularly applicable for igniting both a pilot flame and a main burner flame useful in the production of large continuously flowing volumes of inert gas as may be required for bulk drying and mild gasification of substantially continuously flowing streams of coal or other carbonaceous free flowing bulk solids. The on-gas streams are recycled meaning that the inerts, (i.e., nitrogen from air, carbon dioxide from the combustion reaction and water vapor from either the combustion reaction or from drying) remain except for venting, and are utilized repetitively.